Ind. Eng. Chem. Res. 2001, 40, 393-397
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Fluidization of Waste-Wood Particles with Mechanical Agitation of the Bed Joaquı´n Reina, Enrique Velo, and Luis Puigjaner* Department of Chemical Engineering, ETSIIB, Universidad Polite´ cnica de Catalun˜ a, Diagonal 647, 08028 Barcelona, Spain
A study on the hydrodynamic behavior of waste-wood particles and their fluidization with mechanical agitation of the bed is carried out. Results show that the source of wood particles has an important influence on their hydrodynamic behavior. It is also shown in this work that it is feasible to fluidize such particles by combining the action of gas flow with the mechanical agitation of the bed. Agitation velocities over 30 rpm are not recommended for the fluidization of these kinds of systems. Introduction
Table 1. Basic Properties of the Particles Employed in the Study
Because of the large amount of wood generated as waste from different sources, this material currently returns to capture its role as a source of primary energy. Thermochemical processes such as combustion, pyrolysis, and gasification in fluidized-bed reactors constitute promising alternatives in the production of heat and electrical energy from waste wood. Despite this, studies on the fluidization of wood particles are scarce because they present an unstable hydrodynamic behavior that makes experimentation difficult. Research work carried out on these types of particles focuses on systems of mixtures with sand or other compounds (Mascarenhas et al.1) because of the problems of attaining acceptable fluidization if only wood particles are considered. There are only a few works dedicated to the study of the system hydrodynamics in this specific type of equipment (Olazar et al.2). Reina et al.3 have studied the properties and hydrodynamic behavior of five different types of waste-wood particles at different temperatures. They conclude that the hydrodynamic behavior and properties of wastewood particles are different according to the origin of wood and the thermal treatment at which they are submitted. The same study concludes that these kinds of particles could be classified as Geldart C type powders. According to Walas,4 rat holes or channeling of the bed appears when the coherence of the particles under fluidization is so strong that the gas passes between grouped particles crossing the bed in branched streams. In this condition, the pressure drop is much lower than that corresponding to the weight of the fragmented bed, which provokes an unstable hydrodynamic behavior. This behavior is shown when particles are very small or they do not contain a sufficient proportion of the bigger material. A high value of the relationship height/ diameter of the bed favors the perforation, whereas an increase of the speed of the gas tends to compensate this effect. The breaking-up of the bed is originated when bubbles of gas are so large that they fill the entire transversal section of a small volume. This phenomenon takes place * To whom correspondence should be addressed. E-mail:
[email protected].
nature of the particle
dp (mm)
Fap (kg/m3)
φap
0
1
type of wood
forest demolition machine furniture pallets
1.14 1.48 1.63 1.57 1.69
621 759 529 621 505
0.69 0.35 0.24 0.32 0.33
0.47 0.56 0.70 0.62 0.65
0.61 0.67 0.77 0.76 0.74
hardwood hardwood softwood softwood softwood
when particles are too large or they do not contain a sufficient portion of fines. The pressure drop is irregular and generally much greater than those in regular conditions. Again, the breaking-up of the bed is favored by the increase in the relationship height/diameter of the bed and disappears upon reduction of the gas velocity. The breaking-up of the bed and perforations are undesirable effects, not only because they originate undesirable pressure drop variations but also because they reduce the contact between the phases present in the system. The properties of the particles affect to a large extent the general behavior of the bed and the quality of the fluidization. The objective of this work is to analyze whether the types of particles could be fluidized and furthermore to determine which are the characteristic specific conditions to achieve fluidization. The fluidization through mechanical agitation of the Geldart C particles has been qualitatively studied not only by Brekken et al.6 but also by Nezzal et al.7 They arrived at the conclusion that particles corresponding to this group can be fluidized by mechanical agitation with lower energy consumption. Experimental Section The system under study is formed by waste-wood particles from different origins (forest clearing, construction demolition, slot machines, furniture, and pallets). Table 1 reports their physical characteristics. Table 2 shows the distribution in four fractions of each type of particle according to its origin. Experimental Setup for Hydrodynamic Tests. To study the hydrodynamic behavior of the particle systems, two types of columns were used in order to corroborate the results obtained. On the one hand, a
10.1021/ie000048l CCC: $20.00 © 2001 American Chemical Society Published on Web 11/23/2000
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Table 2. Particle Size Distributions in Terms of Mass Fraction
woods
X1 (2-1.20 mm)
X2 (2-1.20 mm)
X3 (0.71-0.315 mm)
X4 (0.315-0.177 mm)
forest demolition machine furniture pallets
0.16 0.24 0.26 0.24 0.25
0.34 0.59 0.55 0.56 0.64
0.29 0.14 0.19 0.18 0.11
0.2 0.03 0.02
Figure 2. Type and basic dimensions (mm) of the stirrer.
Figure 1. Installation used for the study of fluidization with mechanical agitation of the bed. (1) Glass column: i.d. ) 53 mm, h ) 750 mm. (2) Lapple cyclone. (3) Rotameter. (4) Differential gauge. (5) Regulation valve of the air flow. (6) Driver. (7) Stirring. (8) Speed control. (9) Ammeter. (10) Voltmeter.
glass column (51 mm i.d. and 1000 mm height) provided with a metallic fabric distributor mesh of 2304 orifice/ cm2 was used. Otherwise, the column used was of 76 mm i.d. and with the same type of distributor. Experimental Setup for Fluidization with Mechanical Agitation. The equipment used for the analysis of the effects produced by mechanical agitation on the particle bed is shown in Figure 1. The main part is a glass column (51 mm i.d. and 750 mm height) with a Lapple cyclone coupled at the top of the column for the separation of fine particles. A water column manometer is used to measure the pressure drop across the bed. The air pressure is also measured at the column inlet by a Bourdon gauge. Calibrated rotameters are provided for measuring the air flow (0-2 and 0-11 m3/ h), and ball valves are installed (12.5 mm i.d.) for the regulation of the air flow. The gas distributor consists of a metallic mesh of 2304 orifices/cm2. The working fluid was dry air at room temperature, and the flow in all experiments oscillated between 0.4 and 6 m3/h. The agitation system was provided with an electrical driver (Heidolph type RZR-1) with speed control from 30 to 250 rpm. A voltmeter and an ammeter were installed for the simultaneous measurement of voltage and current; the voltage was measured across the driver and the intensity in series with the ammeter. A semi-inverted-anchor type of agitator was designed and built, taking into account the characteristics of the flowing particle system. The stirrer height was the same as that of the bed (l/d ) 1) and the diameter of the pallets the same as the average diameter of the particles. Figure 2 shows the details of the agitation system.
With this type of agitator the surface contact between the phases is improved because the bed remains split into cells. Methods of Fluidization. Two general procedures were pursued for the determination and analysis of the main characteristics of the fluidization of the different particles employed in this work. In the first one, the rotating speed was maintained constant through the experiment. In this case, stirring is started first, and then the gas flow is gradually increased to a maximum value depending on the type of particle. Then, reverse operation takes place, measuring for each flow the pressure drop in the bed, the intensity and voltage, and the bed expansion from the beginning to the end of the stirring process. Rotating speeds have been chosen in the range between 30 and 115 rpm. The second one had the purpose of observing how the rotation speed influences the fluidization quality of these types of particles. It consisted of carrying the bed until the state of fluidization at low speed (30 rpm), maintaining constant the gas flow in all cases, then increasing step by step the stirring velocity, and registering simultaneously the pressure drop across the bed and the voltage and current fed to the driver of the rotating system. The reported power in this case corresponds with the electrical power consumed by the driver during its operation, which is determined from voltage and current measurements. Results and Discussion Hydrodynamic Behavior. Experimental results show that the hydrodynamic behavior of the different wood particles is similar in all cases to the behavior of particles of group C given by Geldart.5 The particles tend to interlace, and while the flow of the gas is increased, a channeling effect appears from the surface of the distributor up to the surface of the bed. Otherwise, in the case of wood originating from machines and pallets (softwood), the whole bed is displaced upward in plug-flow fashion. All other types of particles fluidize well when the gas flow is increased.
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Figure 3. Hydrodynamic behavior shown by the particles classified as softwood.
Figure 4. Hydrodynamic behavior shown by the particles classified as hardwood.
This behavior can be better explained by looking at Figures 3 and 4. As can be observed, the nature of the particle plays a fundamental role. In the case of wood particles whose origin is the demolition of buildings (hardwood), after having reached the maximum bed expansion, following increasing gas velocities, channeling or rate holes are formed. A further increase of the speed of the gas results in uniform fluidization of particles because the bed becomes stable and an observable free surface is maintained, reaching what is designated as fluidization in the dense phase. Otherwise, in the other system (softwood), after having reached the maximum bed expansion, this is broken up and behaves as a plug-flow regime. The different stages appearing during fluidizations of the systems under study are indicated in Figures 3 and 4. Therefore, it can be concluded that the origin of the particles plays a fundamental role in their hydrodynamic behavior. What causes difficulty in the fluidization of these systems depends to a large extent on the physical characteristics and origin of the particles. These characteristics can be summarized as follows: (1) low density; (2) very irregular shape (depending also on the way of pretreatment); (3) associated bed low density; and (4) soft and porous material of fibrous texture. All of these characteristics contribute to a high coherence of the particles as well as to a great porosity of the bed, thus yielding electrostatic charges due to surface rubbing and interlacing produced during the stepping up of the gas flow. As a consequence, the
Figure 5. Hydrodynamic behavior of the waste-wood particles through mechanical agitation. Forest wood classified as hardwood.
training of the agglomerated particles is provoked which drags other particles, thus facilitating the plug-flow behavior (breakup of the bed), in some instances, or channeling or perforations, in others. This causes the unusual shape of the fluidization curves, especially at the stage of defluidization, otherwise typical of particles corresponding to group C (Geldart5). The values of the basic properties for the different types of particles are reported in Table 1. Those particles coming from coniferous woods or softwood contribute to higher bed porosity but show lower apparent density values and greater mean diameters under the same sieving conditions, all of which correlates well with the hydrodynamic plug-flow behavior already discussed. Furthermore, it can be also appreciated that the bed porosity increase is smaller in the expanded bed for hardwood types of particles. The porosity of the expanded bed reaches 0.77 for particles originating from softwood, whereas a much lower value is seen in the case of hardwood (0.67). This favors further channeling and reduces the bed pressure drop, which helps to explain the behavior in the defluidization curves. The fractional distribution of particles is shown in Table 2. It can be observed that in the case of machine wood and pallets, classified as softwoods, the fraction of fines is missing, which corroborates the observation of Walas4 that a breaking-up of the bed is expected in their hydrodynamic behavior. The other types of particles form channels, and with the increase of the gas velocity, fluidization in the dense phase is reached. Characteristics of the Fluidization with Mechanical Agitation of the Bed. First Method of Fluidization. Experimental results demonstrate, as can be observed in Figures 5 and 6, that the waste-wood particles regardless of their origin can be fluidized by the combined action of the gas flow and the mechanical agitation of the bed. At higher gas flow velocity, only hardwood fluidizes. When mechanical agitation of the bed is started, combined with the gas flow action, fluidization in the dense phase is achieved with all types of particles regardless of their origin. However, it is worth noting that there are differences in the way that both types of particles fluidize. Particles classified as hardwood behave as a low-viscosity fluid, originating a vortex of particles in the center of the bed, thus corroborating the observations made by Kozulin and Kulyamin.8 Otherwise, those particles classified as softwood show the behavior of a high-viscosity fluid.
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Figure 6. Hydrodynamic behavior of the waste-wood particles through mechanical agitation. Wood pallets classified as softwood.
Figures 5 and 6 show a sample of the experiments carried out with the first procedure. The hydrodynamic behaviors of both types of particles are quite different. When hardwood particles are used, channeling through the bed (rat hole) is eliminated by mechanical agitation of the bed, which reduces the bed porosity and allows the pressure drop increase at increasing gas flow velocity until the state of fluidization in the dense phase is reached. The level of fluidization is higher because of the agitation of the bed. Here, the plug-flow regime happens first, and the bed porosity is reduced, causing an increase in the pressure drop, which augments with the gas velocity. When agitation is present, the entrained particles in plug flow are disengaged, thus increasing the bed porosity, and as a consequence the pressure drop decreases, reaching in this case lower levels than those observed without mechanical agitation. With increasing gas velocities, the state of fluidization is finally reached. The behaviors presented in these figures correspond to the defluidization stage of the particles, where a change in the slope can be observed when agitation is present. Therefore, it can be concluded that the use of mechanical agitation in these systems of particles causes an increase or decrease of the bed porosity depending on the nature of the particles, contributing in all cases to reaching the state of fluidization. Another characteristic that differentiates the hydrodynamic behavior of both types of particles when mechanical agitation is used is the influence that the height of the bed has. The analysis carried out in a series of experiments demonstrates that softwood or coniferous particles are more sensitive to this influence than the particles whose origin is hardwood or leafy. To achieve an acceptable fluidization of softwood types, it is recommended to use a bed height just over the location of the stirring. This was experimentally confirmed by using a bed height well over the stirring position, which resulted in the segregation of the upper part of the bed in plug flow while the rest of the bed remained fluidized. Another characteristic associated with the mechanical agitation of the bed is that the power consumed by the driver to accomplish the agitation work of these particles is relatively low and remains constant after reaching fluidization. This fact agrees with the results reported by Brekken et al.6 and Nezzal et al.7 Figures 7 and 8 show the behavior of the electrical power consumption during stirring at different agitation velocities for
Figure 7. Behavior of the electrical power consumed during the agitation of the waste-wood particles. Forest wood.
Figure 8. Behavior of the electrical power consumed during the agitation of the waste-wood particles. Wood pallets.
different types of particles as a function of the gas flow velocity. Vertical lines in both figures identify the zones at which the power consumption tends to stay constant. It can be observed that with an increase of the agitation speed this line is displaced toward smaller gas velocities. This observation permits us to conclude that the increase of the agitation speed reduces the gas velocities at which the fluidization state is reached. The total power consumption in the bed (PTC) is the sum of the power consumed by the gas (proportional to the product of the gas flow by the pressure in the bed) and the necessary agitation power (from voltage and intensity measurements). However, as can be seen in Figure 9, the PTC shows the same trend as that of the electrical power consumed in the agitation of the bed alone, given that in this case the power taken by the gas is relatively small. Here again, the nature of the particles influences the observed behavior. Particles classified as hardwood show a smaller consumption than that of softwood particles despite the lower density of the latter. This can be attributed to the fact that the hardwood particles tend to flow more easily given that they are less fibrous. However, when we compare the dissipated total power for particles of forest and demolition, both classified as hardwood, we observe a substantial difference in power consumption that can be attributed to their difference in density. Second Method of Fluidization. In the development of the experiments with the second procedure, it was observed that for speeds of over 50 rpm agitation
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For an adequate fluidization of these systems, the agitation speed should not be greater than 30 rpm. The nature of the particles plays a fundamental role in their hydrodynamic behavior. The hydrodynamic behavior shown by these types of particles follows the pattern of group C particles in the Geldart classification. Substantial differences are found depending on the nature of the particles that can be basically characterized as hardwood or softwood. Acknowledgment This work was financed in part by the Commission of the European Communities (ECSC Program: Contract 7220-ED-081). J.R. acknowledges the financial support from the Fundacio´n Caja Madrid. Figure 9. Behavior of the total power consumed vs the gas velocity for the different types of particles at 30 rpm.
Nomenclature List of Symbols d ) bed diameter, mm dp ) mean diameter of particle by sieving, mm H ) height of column, mm I ) current, A l ) bed length, mm ∆p ) pressure drop across the bed, Pa Pe ) electric power, W PTC ) total power consumed, W V ) voltage, V Xi ) fraction in bulk of the particle i Greek Symbols
Figure 10. Variation of the electrical power and the pressure drop in the bed at increasing stirring speed for the waste wood. Machine wood.
prevails over fluidization phenomena regardless of the nature of the particles and gas velocity. At this point, particles follow the path dictated by the movement of the stirrer, and the fluidized-bed characteristic flow ceases. It can also be observed in Figure 10, that as the speed of rotation increases the pressure drop slightly decreases. Thus, an increase of the stirrer electrical power consumption occurs. Therefore, a boundary could be established between the domains of both mechanisms depending on the stirring velocity. During the experiments carried out with different types of wood particles, it also could be observed that a substantial number of wood particles would remain attached to the surface of the stirring mechanism even when the gas flow was stopped. This indicated that significant electrostatic charges are developed in the agitator by the action of the gas flow, which should be taken into account in the actual system performance on a larger scale. Conclusions The experimental results carried out demonstrate that independently from their origin the waste-wood particles can be fluidized through the combined action of the gas flow and the mechanical agitation of the bed.
φap ) shape apparent factor Fap ) apparent particle density, kg/m3 Ff ) density of the fluid, kg/m3 0 ) porosity of the bed without expansion 1 ) porosity of the expanded bed µ ) viscosity of the gas, Pa/s
Literature Cited (1) Mascarenhas, F.; Gulyurtlu, I.; Cabrita, I. Fluidization behaviour of binary wood/sand mixtures. In Fluidization and Fluid/Particle Systems; Casal, J., Arnaldos, J., Eds.; Universitat Polite`cnica de Catalunya, Gerona, Spain, 1990; p 31. (2) Olazar, M.; San Jose´, M. J.; Llamosas, R.; Bilbao, J. Hydrodynamic of sawdust and mixture of wood residues in conical spouted beds. Ind. Eng. Chem. Res. 1994, 33, 993-1000. (3) Reina, J.; Velo, E.; Puigjaner, L. Fluidization of scrap-wood material: The influence of the degree of thermal decomposition on the hydrodynamic properties. Ind. Eng. Chem. Res. 1999, 38 (8), 3115-3120. (4) Walas, S. Reaction Kinetics for Chemical Engineers; McGrawHill: New York, 1959. (5) Geldart, D. Types of fluidization. Powder Technol. 1973, 7, 285-292. (6) Brekken, R. A.; Lancaster, E. B.; Wheelock, T. D. Fluidization of Flour in to Stirred, Aerated bed: Part I. General Fluidization Characteristics. Chem. Eng. Prog. Symp. Ser. 1970, 66 (101), 81. (7) Nezzal, A.; Release, J. F.; Guigon, P. Fluidization Behavior of Very Cohesive Powders Under Mechanical Agitation. Proceedings of the 8th Engineering Foundation Conference on Fluidization, Tours, France, May 14-19, 1995. (8) Kozulin, N. A.; Kulyamin, A. F. Int. Chem. Eng. 1965, 5, 157.
Received for review January 12, 2000 Revised manuscript received September 27, 2000 Accepted September 27, 2000 IE000048L
